1. Field of the Invention
The present invention relates to a solid-state imaging device, and in particular to a solid-state imaging device formed by stacking a plurality of substrates and a manufacturing method for the solid-state imaging device. The present invention also relates to an electronic device in which the solid-state imaging device is used.
2. Description of the Related Art
Solid-state imaging devices are roughly divided into charge-transfer solid-state imaging devices represented by CCD (Charge Coupled Device) image sensors and amplification solid-state imaging devices represented by CMOS (Complementary Metal Oxide Semiconductor) image sensors.
Each pixel forming a CCD solid-state imaging device is composed of a light sensing section that is formed by a photodiode and that generates a signal charge in response to received light, and a vertical transfer resistor in a CCD structure that transfers the signal charge generated by the light sensing section in the vertical direction. A vertical transfer resistor section is formed for each column of the light sensing section arranged two-dimensionally, for example. A horizontal transfer resistor in a CCD structure is provided in a stage subsequent to the vertical transfer resistor. An output circuit is provided in a stage subsequent to the horizontal transfer resistor. In the CCD solid-state imaging device configured as described above, the signal charge generated by the light sensing section is read out for each row by the vertical transfer resistor to be transferred in the vertical direction, and is then transferred horizontally by the horizontal transfer resistor to the output circuit. The signal charge transferred to the output circuit is amplified to be output as a pixel signal.
Each pixel forming a CMOS solid-state imaging device is composed of a light sensing section made of a photodiode, a floating diffusion node that reads out a signal charge generated by the light sensing section, and a plurality of MOS transistors. The plurality of MOS transistors include a transfer transistor, a reset transistor, an amplification transistor, and (as necessary) a selection transistor. Each MOS transistor is connected to a desired wiring layer, of a multiplicity of upper wiring layers. In the CMOS solid-state imaging device, the signal charge generated by and accumulated in the light sensing section is read out by the floating diffusion node for each pixel. Then, the signal charge read out by the floating diffusion node is amplified by the amplification transistor, and is selectively output as a pixel signal to a vertical signal line formed in the multiplicity of wiring layers by the selection transistor.
The size of the solid-state imaging devices has been reduced in recent years. For example, Japanese Unexamined Patent Application Publication No. 6-291355 discloses a solid-state imaging device with a stacked structure in which MOS transistors are formed in a single-crystalline silicon substrate and a TFT photosensor is formed on top of the single-crystalline silicon substrate on the light incident side via an insulation film. In the solid-state imaging device, the TFT photosensor and the MOS transistor are formed in different layers, which reduces the size of the device without decreasing the light reception area.
Manufacture of the solid-state imaging devices is subjected to a process constraint that only a low-temperature process is applicable after formation of wires. Therefore, in the solid-state imaging devices, it is necessary to first form impurity regions forming the light sensing section and the MOS transistors in a substrate by ion implantation, and to finally form wires to be connected to the substrate or the desired MOS transistors. In the solid-state imaging device with the stacked structure discussed above, also, contact portions and wires to be connected to each layer are formed after each layer in which desired impurity regions are formed is stacked.
For example, in order to implement an electronic shutter function for resetting a signal charge accumulated in the light sensing section in the solid-state imaging device with the stacked structure, it is necessary to form wires and contact portions necessary for the electronic shutter function on top of the light reception surface because of the process temperature constraint. In this way, however, the light reception area may be decreased with the wires and the contact portions formed on top of the light reception surface, although the size of the device may be reduced by employing a three-dimensional structure. Therefore, it is necessary to equivalently increase the pixel area in order to maintain the light reception area, as a result of which the size of the device may not be reduced.